Circuit panels having screened circuitry are well known in the art. Typically a conductive ink, such as a carbon or silver ink, is screened onto the surface of the circuit panel in a desired configuration. The substrate used for the panels, which are generally referred to as circuit boards, may be a rigid member or a flexible member also known in the art as membrane or flexible circuits. The term "circuit panel" as used in this application encompasses the various types of substrates. These circuit panels are used in a variety of applications.
Depending upon the the type of product in which the circuit panel is used, it is often desirable to include devices such as light emitting diodes (LEDs) as indicia, for example, that a particular circuit is energized. To extend the life of such devices, it is generally desirable to reduce the voltage to the LED members by use of resistors. These resistors may be discrete elements or may be screened onto the surface of the substrate. Both leaded and surface mountable resistors having specific resistance values are readily available for mounting to circuit panels, thus enabling one to uniformly select the desired resistance to be used with the LEDs. Printed resistors may also be used on circuit panels. Since these resistors are "created" during the manufacturing of the circuit panel, printing resistors with the desired electrical performance characteristics presents problems not associated with discrete component elements.
The value of a printed resistor depends, inter alia, upon the resistance of the ink used, the shape of the resistor and the amount of "real estate" available on the panel for the resistor. The available space determines the amount of a conductive ink that can be disposed between discontinuous portions of the circuit path electrically connected to the LED or other device. The value of the resistance also is affected by a number of processing variables, such as the mesh of the screen, the percentage ratio of conductive material to filler in the ink, the thickness of the deposited layer, and the viscosity of the ink, which may change during the course of manufacturing the circuit panel primarily owing to evaporation of the solvent. As a result of these variables, circuit panels screened at the beginning of the manufacturing process may have resistors of a different value than circuit panels screened at the end of the manufacturing cycle. The finished circuit panels, therefore, may have LEDs of varying intensity and protection from high voltage.
In order to maintain an acceptable tolerance range for the electrical characteristics of the resistors, it is necessary to monitor the manufacturing process for circuit panels and make adjustments when necessary. Heretofore, adjustments to the value of the resistors was accomplished by reformulating or remixing the ink or by using a different size mesh screen or shape of resistor with the more viscous ink. These methods, however, require the screen to be cleaned or the apparatus taken apart to remove and replace the screen, which may require considerable down time for the equipment.
It is desirable, therefore, to have a method for on-line adjustment of the resistance value and one that may quickly be adjusted by the operator of the equipment.